1. Field of the Invention
The present invention relates to a process for removing contaminants from polyesters, especially post-consumer polyesters.
2. Description of the Related Arts
The recycling of plastics has become an important concern over the last twenty years. The ability to recapture plastics after consumer use and turn the material into another useful article saves landfill space and conserves resources.
Polyesters such as polyethylene terephthalate, or PET, have found extensive commercial use. One such use is in the beverage container industry. However, unlike glass bottles which can be easily cleaned and refilled, plastic containers can be quite difficult to clean. For example, special sensors should be used for detecting the presence of contaminants. In addition, due to flavor absorption into the plastic, the container must be used for holding the same product when refilled. Because of these and other difficulties, plastic containers are generally recycled instead of refilled. Recycling generally involves shredding the plastic bottle and removing contaminants therefrom. The shredded plastic or flakes can then be remelted and extruded into the shape of a new container.
One of the known contaminant-removal processes involves subjecting the polyester material to a high temperature inert gas stream whereby the contaminants contained within the material are effectively removed by the gas stream. But, this technique suffers from a drawback in that the molecular weight of the polymer is increased by such a treatment. In addition, the molecular weight increase varies with respect to the polymer particle size. Therefore, the molecular weight increase will not be uniform or consistent, thereby decreasing the quality of the polymer. This increase and broadening in the average molecular weight thus complicates the recycling process.
One possible method for avoiding an increase in average molecular weight while removing contaminants is to depolymerize the plastic by the use of superheated steam and then carrying out solid state polymerization to return the polymer to the original molecular weight. The steam treatment will also remove the contaminants. However, such a method is complicated in that a molecular weight decrease and increase step are both required. This is especially problematic from the viewpoint of process control since the molecular weight of the solid polymer can not be directly measured during the treatment. Moreover, variations in particle size distribution and resin composition among different manufacturers' products will require different process treatment conditions. Such conditions must be determined through essentially trial and error. Correspondingly, if the product is of unknown origin, or if it is a mixture of polymers, it is difficult, if not impossible, to correctly predict and carry out the correct temperatures and residence times so as to return the polymer to its original molecular weight. Thus, this method is not convenient and is difficult to control.
Accordingly, the prior art has not yet developed a convenient method for removing contaminants from polyesters without increasing the average molecular weight of the polymer.